Wind turbine blades are connected to wind turbine low speed shaft through a rotor hub. The rotor hub is rigidly attached to the wind turbine low speed shaft so that they rotate together when the wind turbine is in use.
Blade connector assemblies are known in the art for releasable attachment of the blades to the rotor hub in a wind turbine. Known blade connector assemblies typically comprise a number of bushings having internal threads. Bushings are usually arranged in at least one circular row or ring structure that is embedded into a blade portion such as the blade root. The bushings are therefore distributed along a circumference of the surface of said blade portion.
Bushings are adapted for threadably receiving corresponding threaded bolts or studs which, in turn, are attached to a portion of the rotor hub. This allows releasable securing of the blade to the hub.
U.S. Pat. No. 4,915,590 discloses one example of blade connector assembly. A multiplicity of rods is embedded around the periphery of the blade root parallel to the axis of the blade. During a bonding process, a substantial length of the rod is coated with a releasing agent to prevent it from bonding to the blade root except at the securement end out toward the formed blade portion. This solution involves time consuming bushing embedding operations as they entail accurate positioning of the multiplicity of rods.
It is also known to manufacture the blades for a wind turbine in one or more moulds in a way that the blades have integrally moulded bushings for attachment to the hub. In this respect, solutions are known consisting in providing an entire ring or ring segments of bushings integrated in the blade.
Document US2007065288 teaches a blade connection assembly comprising bushings that are bonded to the wind turbine blade root. The bushings have internal threads for mounting studs for releasable attachment of the blade root to the hub. The blade root is made of two parts that are assembled after hardening. At least one layer of fibre mat is placed in a mould and a foam holder is placed on the fibre mat. The holder has spaced recesses for accommodation of the bushings. At least one first layer of fibre mat is provided covering the recesses before placing the bushings. The outer side of the holder is provided with an adhesive and a shell laminate. Fibre glass strips are arranged between the bushings to extend in the longitudinal direction of the blade.
The main disadvantage of this blade connector assembly is that the fibre-bushing interface has a reduced contact of the metal cylindrical surface of the bushing to the fibres. In this case, for providing a greater fiber-bushing contact the bushings should be slightly roughened or with protrusions on the external surfaces such as barbs or flanges. This undesirably increases the costs of the blade connector assembly and therefore the overall costs of the installation.
Document WO03057457 discloses a wind turbine blade connector assembly for connecting a blade portion to a hub portion in a wind turbine rotor. The connector assembly comprises a number of bushings arranged in at least a first row along a circumference and suitable for receiving corresponding studs. Each bushing is in the form of a wedge shaped elongated body with first and second end portions, substantially planar upper and lower parallel faces and concave and convex cylindrical lateral faces. Several bushings may be arranged in parallel to allow the convex lateral faces to engage the concave lateral faces and such that the longitudinal axes of the elements extend in parallel along a curve in a plane perpendicular to the longitudinal axes of the embedding elements.
The curved lateral faces provided in this solution are required since the bushings are made of the same composite material as the blade. This results in a lower rigidity of the blade connector assembly. Otherwise, the geometry of the bushings in this solution with curved faces would be difficult and expensive to machine if they were made of metal.
In addition, it has been found that a critical point in a wind turbine blade connector assembly is the interface surface between the blade laminate and the ring structure of bushings. The above solution has the disadvantage that the concave and convex lateral faces are in close contact to each other and therefore no fibres can be provided between the bushings. This results in a poor load transfer between the bushings and root blade.